December 11, 2006

A surprisingly recent instance of human evolution has been detected among the peoples of East Africa. It is the ability to digest milk in adulthood, conferred by genetic changes that occurred as recently as 3,000 years ago, a team of geneticists has found. The finding is a striking example of a cultural practice — the raising of dairy cattle — feeding back into the human genome.

It also seems to be one of the first instances of convergent human evolution to be documented at the genetic level. Convergent evolution refers to two or more populations acquiring the same trait independently.

Throughout most of human [pre]history, the ability to digest lactose, the principal sugar of milk, has been switched off after weaning because there is no further need for the lactase enzyme that breaks the sugar apart. But when cattle were first domesticated 9,000 years ago and people later started to consume their milk as well as their meat, natural selection would have favored anyone with a mutation that kept the lactase gene switched on.

Such a mutation is known to have arisen among an early cattle-raising people, the Funnel Beaker culture, which flourished some 5,000 to 6,000 years ago in north-central Europe. People with a persistently active lactase gene have no problem digesting milk and are said to be lactose tolerant. Almost all Dutch people and 99 percent of Swedes are lactose-tolerant, but the mutation becomes progressively less common in Europeans who live at increasing distance from the ancient Funnel Beaker region.

Geneticists wondered if the lactose tolerance mutation in Europeans, first identified in 2002, had arisen among pastoral peoples elsewhere. But it seemed to be largely absent from Africa, even though pastoral peoples there generally have some degree of tolerance.

A research team led by Sarah Tishkoff of the University of Maryland has now resolved much of the puzzle. After testing for lactose tolerance and genetic makeup among 43 ethnic groups of East Africa, she and her colleagues have found three new mutations, all independent of each other and of the European mutation, which keep the lactase gene permanently switched on. [More]

Similarly, the mutation for fair skin color is different in Europeans than in Northeast Asians.

3 comments:

Actually, lactase persistence in Europeans occurs on a haplotype block of ~1 million DNA units.One founder multiplied 100's of millions of times over, and his particular alleles present in this essentially unchanged block are what was expanded.All sorts of other characters would have been carried along with the lactase-related alleles under selection.

What if such populations expanded nepotism towards moderate ranges of genetic distance, beyond cousins,but this turned out to be a manipulation of a lactobacteria, building on the1 million DNA unit block,(Lactase Persistence A)getting it treated as if it represented the totality of the variable genes?How can nepotism and reduced suspicion and hostility ever widen its circle in such degrees? That is, where genetic causation is to have a great role, and has to start from nothing, and not be free-rided out of its chance to exist.Would some heritable preference for such a block as long as the above, be enough to generate the expanded nepotism and trust tendencies, strictly regarding genetic distance,or would there have to be some additional push as from a microbial manipulation, such as the above-suggested?These further speculations arose from the subject of possible genetic differences on the attitudes mentioned.

Anyone who now says natural selection is insignificant in man during the age past the divergence of the great races, has now some facts to reckon with, which decidedly contradict his feeling of what must be.

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